Bad request when posting to OData Data Entity in Dynamics 365

Question

I've created a public Data Entity in dynamics with the following fields:

I keep getting a bad request response, but I'm not sure why. I've tried to make a POST request in two ways:

1.

HireAction hireAction = new HireAction() { CompanyName = "DEMF", MovieId = "DEMF-000000014", HireActionStatus = "Created" };
                string jsonMessage = JsonConvert.SerializeObject(hireAction);
                using (HttpClient client = new HttpClient())
                {
                    HttpRequestMessage requestMessage = new
                    HttpRequestMessage(HttpMethod.Post, "MyDynamicsEnvironmentName/data/HireActions?cross-company=true");
                    requestMessage.Content = new StringContent(jsonMessage, Encoding.UTF8, "application/json");
                    requestMessage.Headers.Add("Authorization", AuthResult.AuthorizationHeader);
                    HttpResponseMessage response = client.SendAsync(requestMessage).Result;
                    if (response.StatusCode == System.Net.HttpStatusCode.OK)
                    {
                        //Logic
                    }
                }

            var url = "MyDynamicsEnvironmentName/data/HireActions?cross-company=true";
            var req = HttpWebRequest.Create(url);
            req.Method = "POST";
            req.ContentType = "application/json";
            req.Headers["Authorization"] = AuthResult.AuthorizationHeader;

            HireAction hireAction = new HireAction() { CompanyName = "DEMF", MovieId = "DEMF-000000014", HireActionId = "12345", HireActionStatus = "Created" };
            var jsonSettings = new JsonSerializerSettings
            {
                DateTimeZoneHandling = DateTimeZoneHandling.Local
            };
            var postString = "CompanyName='DEMF'" + "&MovieId='DEMF-000000014'" + "&HireActionId=132&HireActionStatus='Created'";
            var data = JsonConvert.SerializeObject(postString, jsonSettings);
            var bytes = Encoding.Default.GetBytes(postString);
            var newStream = req.GetRequestStream(); 
            newStream.Write(bytes, 0, bytes.Length);
            newStream.Close();

            using (var resp = req.GetResponse())
            {
                var results = new StreamReader(resp.GetResponseStream()).ReadToEnd();
            }

Some keypoints:

-Of course you'd replace MyDynamicsEnvironmentName with the URL for the environment. The URL is correct and verified however, by the fact that GET requests do work

-The Authresult.AuthorizationHeader contains a valid token, also validated by working GET requests

As said before, both of these result in a bad request. Does someone know what is wrong or missing?